There is considerable interest in developing systems for accurately measuring the thickness and/or composition of thin films. The need is particularly acute in the semiconductor manufacturing industry where the thickness of these thin film oxide layers on semiconductor substrates is measured. To be useful, the measurement system must be able to determine the thickness and/or composition of films with a high degree of accuracy. The preferred measurement systems rely on non-contact, optical measurement techniques, which can be performed during the semiconductor manufacturing process without damaging the wafer sample. Such optical measurement techniques include directing a probe beam to the sample, and measuring one or more optical parameters of the reflected probe beam.
In order to increase measurement accuracy and to gain additional information about the target sample, multiple optical measuring devices are incorporated into a single composite optical measurement system. For example, the present assignee has marketed a product called OPTI-PROBE, which incorporates several optical measurement devices, including a Beam Profile Reflectometer (BPR), a Beam Profile Ellipsometer (BPE), and a Broadband Reflective Spectrometer (BRS). Each of these devices measures parameters of optical beams reflected by, or transmitted through, the target sample. The BPR and BPE devices utilize technology described in U.S. Pat. Nos. 4,999,014 and 5,181,080 respectively, which are incorporated herein by reference.
The composite measurement system mentioned above combines the measured results of each of the measurement devices to precisely derive the thickness and composition of the thin film and substrate of the target sample. However, the accuracy of the measured results depends upon precise initial and periodic calibration of the measurement devices in the optical measurement system. Further, recently developed measurement devices have increased sensitivity to more accurately measure thinner films and provide additional information about film and substrate composition. These newer systems require very accurate initial calibration. Further, heat, contamination, optical damage, alignment, etc., that can occur over time in optical measurement devices, affect the accuracy of the measured results. Therefore, periodic calibration is necessary to maintain the accuracy of the composite optical measurement system.
It is known to calibrate optical measurement devices by providing a reference sample having a known substrate, with a thin film thereon having a known composition and thickness. The reference sample is placed in the measurement system, and each optical measurement device measures the optical parameters of the reference sample, and is calibrated using the results from the reference sample and comparing them to the known film thickness and composition. A common reference sample is a "native oxide" reference sample, which is a silicon substrate with an oxide layer formed thereon having a known thickness (about 20 angstroms). After fabrication, the reference sample is kept in a non-oxygen environment to minimize any further oxidation and contamination that changes the thickness of the reference sample film away from the known thickness, and thus reduces the effectiveness of the reference sample for accurate calibration. The same reference sample can be reused to periodically calibrate the measurement system. However, if and when the amount of oxidation or contamination of the reference sample changes the film thickness significantly from the known thickness, the reference sample must be discarded.
For many optical measurement devices, reference samples with known thicknesses have been effective for system calibration. Oxidation and contamination that routinely occurs over time with reference samples is tolerable because the film thickness change resulting from the oxidation/contamination is relatively insignificant compared to the overall thickness of the film (around 100 angstroms). However, new ultra-sensitive optical measurement systems have been recently developed that can measure film layers with thicknesses less than 10 angstroms. These systems require reference samples having film thicknesses on the order of 20 angstroms for accurate calibration. For such thin film reference samples, however, the changes in film layer thickness resulting from even minimal oxidation or contamination are significant compared to the overall "known" film layer thickness, and result in significant calibration error. Therefore, it is extremely difficult, if not impossible, to provide a native oxide reference sample with a known thickness that is stable enough over time to be used for periodic calibration of ultra-sensitive optical measurement systems.
There is a need for a calibration method for ultra-sensitive optical measurement devices that can utilize a reference sample that does not have a stable or known film thickness.